This invention relates to an ignition coil unit for an internal combustion engine and, more particularly, to an ignition coil unit in which an ignition coil and a power switch for controlling a primary current through the ignition coil are integrally combined into a unit.
FIG. 6 is an electrical circuit diagram of a known ignition coil unit for an internal combustion engine. The ignition coil unit comprises an ignition coil A having a primary coil 2 and a secondary coil 7, and a power switch circuit B having a plurality of electric and electronic circuit components. In FIG. 6, it is also seen that an electric source C and an ignition signal control circuit D are connected to the ignition coil unit.
The power switch circuit B comprises a power transistor 1 for switching a primary current flowing through the primary coil 2 of the ignition coil A, a current limiting circuit 4 and a current detecting circuit 3 for detecting a potential difference generated by the primary current and for transmitting a primary current control signal to the current limiting circuit 4.
FIG. 7 is a sectional side view of a conventional ignition coil unit in which the ignition coil A and the power switch circuit B are integrally combined, and FIG. 8 is a front view of the ignition coil unit illustrated in FIG. 7 before it is filled with insulating resin. In FIGS. 7 and 8, reference numeral 5 designates a casing, 7 is the secondary coil disposed within the casing 5 and concentrically wound around the primary coil 2 and an iron core 8. The iron core 8 is a substantially C-shaped member having a pair of substantially U-shaped members welded together at an end of one of the legs of the "U" positioned in an opposing relationship. An air gap 8a is defined between opposing legs of the "U". Reference numeral 9 is an aluminum, heat dissipating plate attached to the casing 5, the heat dissipating plate 9 having been attached thereto, through an electrically insulating plate 10, the power transistor 1 and the current limiting circuit 4. Reference numeral 11 is a silicone resin hermetically sealing the power transistor 1 and the current limiting circuit 4 for protecting the same and improving heat dissipation, 12 is a connector integrally molded with the casing 5, 13 is a cover for hermetically sealing a power switch cavity 14 in order to ensure the water-resistivity of the power switch circuit including the power transistor 1 and the current limiting circuit 4, and 15 is an electrically insulating resin filled within the casing 5.
Reference numeral 16 is a secondary coil ground line for connecting a first connector terminal 17 and one end of the secondary coil 7, 18 is a source line for connecting a second connector terminal 19 and one end of the primary coil 2, 20 is a control signal line for connecting a third connector terminal 21 and a base terminal 22 of the power transistor 1, 23 is a ground line for connecting a fourth connector terminal 24 and a current limiting circuit terminal 25 of the current limiting circuit 4, and 26 is a collector line for connecting a collector terminal 27 of the power transistor 1 and the other end of the primary coil 2. The control signal line 20, the earth line 23 and the collector line 26 extend through the casing 5 to be connected to the base terminal 22, the current limiting circuit terminal 25 and the collector terminal 27, respectively within the power switch cavity 14.
In the conventional ignition coil unit as described above, the primary current of the primary coil 2 flows through the current detection circuit 3, where the current level is detected as the potential difference upon which a control signal is supplied to the current limiting circuit 4. The current limiting circuit 4 controls, in accordance with this control signal, the primary current flowing through the primary coil 2 of the ignition coil A. In response to this primary current flowing through the primary coil 2, a high voltage to be supplied to a distributor (not shown) is generated in the secondary coil 7 of the ignition coil A.
With the conventional ignition coil unit as described above, the circuit components including the power transistor 1 and the current limiting circuit 4 must be carefully attached to the heat dissipating plate 9 through the insulating plate 10 so that the power transistor 1 and the current limiting circuit 4 are not destroyed. Also, in order that the power transistor 1 and the current limiting circuit 4 are covered with the resin 11 to protect them, the power switch cavity 14 must be provided in the casing 5, preventing a reduction in the dimensions of the ignition coil unit.
Also, since the power switch circuit B is disposed relatively close to the ignition coil A for compactness, the power switch circuit B can be electrically affected by the ignition coil A and erroneously operated.
Further, since the iron core 8 is a substantially C-shaped member having a small gap 8a placed inside of the ignition coil A, the distance between the air gap 8a and the power switch circuit B is small. Therefore, the power switch circuit B is subjected to heat generated at the air gap 8a, a relatively large amount of heat being generated due to a large magnetic reluctance.
Also, although an iron core 8 is provided, a leakage magnetic flux from the ignition coil A may affect the power switch circuit B, causing the power switch circuit B to be erroneously operated.